Cyclic torque pulsation that occurs as a rotor is rotated under a condition where a motor is not energized is caused by the interaction between an iron core and permanent magnets, and is referred to as cogging torque. When a motor having high cogging torque is used, for example, to drive a feed shaft of a machine tool, defects such as creation of moiré will occur. For this reason, there is demand for a motor of lower cogging torque.
However, the amplitude of cogging torque typically has a very small value that, for example, accounts for only 1% of a continuous rated torque of the motor in the machine tool or less, and measurement of the cogging torque is difficult. Conventionally, various suggestions have been put forth about a method for measuring the cogging torque of a motor.
For example, JP 2006-220497 A discloses a cogging torque measuring method by means of a torque meter. The cogging torque measuring method is specifically described with reference to FIG. 5. FIG. 5 shows a schematic configuration of a cogging torque measuring apparatus. A cogging torque measuring apparatus 41 includes a driving motor 50 functioning as a drive means for rotating a motor 42 to be measured, a torque meter 51 functioning as a torque detection means, and an encoder 52 functioning as a rotation angle detecting means. An output shaft 42a of the motor 42 to be measured is connected through a first coupling 43 to a rotating shaft 51a of the torque meter 51 in an integrally rotatable manner, and the rotating shaft 51a of the torque meter 51 is further connected through a second coupling 44 to an output shaft 50a of the driving motor 50 in an integrally rotatable manner. The driving motor 50 is actuated based on a command from a controller 54, to thereby rotate, via the second coupling 44, the torque meter 51, and via the first coupling 43, the motor 42 to be measured in a non-energized state. Then, the torque meter 51 detects a torque generated in the motor 42 to be measured and outputs a torque detection signal corresponding to the detected torque. The encoder 52, which is mounted on the driving motor 50, measures an angle of rotation of a not-illustrated drive shaft of the driving motor 50 to detect an angle of rotation of the motor 42 to be measured, and outputs to the controller 54 an angle detection signal corresponding to the detected angle. The controller 54 determines, based on both the torque detection signal input via a filter 53 from the torque meter 51 and the angle detection signal input from the encoder 52, the cogging torque of the motor 42 to be measured.
Meanwhile, JP 2010-158123 A discloses a method for measuring a cogging torque, in which a value obtained by second order differentiation of an angle of rotation of a rotor while a motor is rotated is multiplied by a moment of inertia of the rotor. This method for measuring a cogging torque is described specifically with reference to FIG. 6. FIG. 6 is a block diagram of a cogging torque detecting apparatus. The cogging torque detecting apparatus 61 includes an inverter 71, a motor 72 to be measured, an encoder 73 functioning as a rotation angle detecting unit, and a dedicated controller 64 functioning as a control apparatus for the motor 72. In this method, an angle of rotation, which is detected by the encoder 73 when the motor 72 is driven through the inverter 71 at a constant torque, is second order differentiated in an angular acceleration calculating unit 81 to find a rotation angular acceleration, and in a multiplier 93, the rotation angular acceleration is multiplied by a moment of inertia 92 of the rotor in the motor 72 to be measured, to thereby calculate a cogging torque.
In the previously-described measuring method shown in JP 2006-220497 in which the driving motor 50 is connected via the torque meter 51 to the motor 42 to be measured, the moment of inertia of the driving motor 50 is required to be much higher than that of the motor 42 to be measured, for measurement of the cogging torque with accuracy. The reason for this is described using mathematical expressions below. An acceleration for a change in velocity caused by the cogging torque of the motor 42 to be measured is taken as a, a moment of inertia of the rotating shaft in the driving motor 50 is taken as Jd, and an output torque of the driving motor 50 is taken as Td. The torque detection signal St output from the torque meter 51, which indicates the sum of the cogging torque transmitted from the motor 42 to be measured through the torque meter 51 to the driving motor 50 and the output torque Td of the driving motor 50, is expressed by Equation (1) (in the equations below, a friction torque of the rotating shaft and a moment of inertia of the torque meter are not taken into account) as follows:St=Jd·α+Td  (1)
Taking into account the acceleration α, which is obtained as α=(a torque exerted on the rotating shaft)/(the moment of inertia of the rotating shaft), Equation (2) is obtained for the acceleration α as follows:α=(Tcog−Td)/(Jm+Jd)  (2)
where Jm indicates the moment of inertia of the motor 42 to be measured, and Tcog indicates the cogging torque of the motor 42 to be measured.
Here, assuming a condition of Td=Tcog, the acceleration α in Equation (2) becomes zero. Substituting this value of α in Equation (1) yields St=Td=Tcog. In this case, the cogging torque can be accurately measured by measuring the torque detection signal output from the torque meter 51. The condition of Td=Tcog, however, is a condition where the change in velocity caused by the cogging torque of the motor 42 to be measured is completely cancelled by controlling the velocity of the driving motor 50. In practice, it is almost impossible to actually establish the condition, and a failure to establish the condition inevitably invites an error in measurement. A way of making the torque detection signal St as close to the cogging torque Tcog as possible is described below. Substitution of Equation (2) into Equation (1) yields the following Equation (3).
                              S          t                =                                                            J                d                            ·                                                                    T                    cog                                    -                                      T                    d                                                                                        J                    m                                    +                                      J                    d                                                                        +                          T              d                                =                                                                      J                  d                                                                      J                    m                                    +                                      J                    d                                                              ·                              T                cog                                      +                                                            J                  m                                                                      J                    m                                    +                                      J                    d                                                              ·                              T                d                                                                        (        3        )            
From Equation (3), it can be understood that St≈Tcog is obtained when both conditions Jd/(Jm+Jd)≈1 and Jm/(Jm+Jd)≈0 are satisfied. To achieve this, Jd should be much greater than Jm.
From the above, it is proved that the moment of inertia Jd of the driving motor 50 needs to be sufficiently greater than the moment of inertia Jm of the motor 42 to be measured in order to perform accurate measurement of the cogging torque. For this reason, when a cogging torque of a large motor is measured, it is necessary that the installed driving motor 50 be much larger than the motor 42 to be measured. This results in a problematic increase in size of a measurement apparatus.
On the other hand, in the measuring method disclosed in JP 2010-158123, because a current is passed through the motor 72 to be measured, a torque pulsation component caused by the passage of the current is also measured together with the cogging torque. This raises a problem that it is not possible for the measuring method to solely measure the cogging torque.